1. Field of the Invention
This invention pertains to the field of electronics and in particular to the field of digital electronics. With greater particularity, this invention pertains to the field of digital signal processing. With greatest particularity, the present invention pertains to a null filter circuit for digital signal processing to prevent erroneous guidance commands in an anti-radiation guided missile (ARM).
2. Description of the Related Art
Anti-radiation missiles are generally passive tracking devices, relying on the radio frequency (RF) energy emitted from a target to generate tracking signals, zeroing out angle errors, and following this energy path to a point of impact upon the target. Radiating targets of interest usually have highly directional energy density patterns in order to achieve small angular resolution for their target tracking purposes.
This is accomplished by focusing the energy with an antenna into a main beam. The focusing process is not perfect, generating lower power density beams known as sidelobes and backlobes which vary in energy density, solid angle, and angular position from the center of the main beam. Included in this beam structure are angular areas where very small amounts of energy are radiated out from the target, known as nulls.
These nulls present a warped phase front which makes the target appear to be emanating from a different position than it""s actual location, and they also allow the ARM to receive signals off surrounding objects (multipath) making the target appear to be in a different location. The missile signal processing may then generate erroneous guidance information steering the missile away from the intended target, causing a miss.
The problem of erroneous missile guidance caused when an anti-radiation missile, which passively tracks radiation emission from a target, encounters a target null, has been solved by the present invention which detects when a null condition exists and momentarily attenuates the guidance error signal so that no guidance commands are issued during the time the null is present.
The invention includes an analog differencing circuit, a comparator, a one-shot multivibrator, a D flip-flop, a clock circuit (variable), three three-input NAND gates, an up/down counter, a multiplying digital to analog converter, two four-input NAND gates, four digital inverters, and an analog output buffering circuit.
Signal inputs include the automatic gain control (AGC) voltage which represents the average power level of the energy received from the target, and the guidance error signal epsilon from which all control commands are generated. The output is a modified epsilon signal which is attenuated when a null is detected, removing the erroneous guidance commands.
The null filter is designed to use the physical property of increasing signal power level as the missile approaches the target. If the received power level begins to drop, and does so at a rapid rate, a target anomaly or null has probably been encountered. By detecting this drop in power level, action may be taken to remove the bad tracking data from the guidance commands allowing the missile to coast, until good tracking data is again received.